Recently, a stereolithographic method and apparatus for manufacturing a stereolithographic three-dimensional object by means of curing a photocurable resin on the basis of data input a three-dimensional CAD system has been put into practical use. This stereolithography technique gets a lot of attentions, because a complicate three-dimensional object, such as a model used for verifying an external design during the course of designing operation, a model used for checking the functionality of a component, or a base model used for making a plastic or metal mold for use in manufacturing a casting mold, can be built readily.
A method using a mold bath is used for general purpose at the time of manufacture of a three-dimensional object by the stereolithography. A method widely adopted as procedures of that method comprises the process steps of: putting a liquid photocurable resin into a mold bath; selectively radiating a spot-shaped UV laser beam, which is controlled by a computer so as to obtain a desired pattern on the surface of the liquid, to thus optically cure the resin to a predetermined thickness to thereby form a cured resin layer; downwardly moving the cured resin layer within the mold bath to cause the photocurable resin liquid in the mold bath to flow above the cured resin layer to thereby form a layer of the photocurable resin liquid; radiating the spot-shaped UV laser beam to the photocurable resin liquid layer, to thereby form a cured resin layer; and repeating the above processes until a stereolithographic three-dimensional object of predetermined shape and size is formed.
However, the above-described conventional method employing the spot-shaped UV laser beam is a so-called stippling method for forming a planar optically-cured pattern by means of radiating, in a moving manner; one spot-shaped laser beam onto the surface of a photocurable resin; and has problems of building involving consumption of much time and low productivity. Moreover, a UV laser system used as the light source is extremely expensive, which makes the stereolithographic apparatus of this type expensive.
With a view toward solving the previously-described drawbacks of the conventional technique, a stereolithographic method has been proposed (see JP-A-4-305438), wherein a linear exposure mask where optical shutters, each being capable of controlling shielding of light in a microdot area, are continuously arranged in a line is used, to thus control the optical shutters according to predetermined horizontal cross-sectional profile data by means of scanning the exposure mask in a direction orthogonal to the arranged direction of the optical shutters, thereby sequentially forming one layer of optically-cured resin layer. In the case of adoption of this method, using an expensive UV laser system as the light source is not always required, and an inexpensive light source such as an ordinary UV lamp can be used. When compared with the conventional method employing the spot-shaped UV laser beam, this method enables an increase in building rate. However, this method is a scheme where lines of linear optically-cured sections are formed one by one in the scanned direction of a photo-mask and where a cross-sectional profile pattern for one layer is formed by means of repeating, a plurality of times, formation of the linear optically-cured section. If the scan speed of the photo-mask is increased, an optically-cured section that is sufficiently cured cannot be formed on a per-line basis. Therefore, the photo-mask must be slowly scanned. Moreover, this method is to form a planar optically-cured layer by means of making an optically-cured section one after another on a per-line basis, which involves consumption of much time for building the entire article. Therefore, the building rate cannot be said to be sufficiently fast, and sufficient satisfaction cannot be achieved in view of productivity.
Another known method for manufacturing a stereolithographic three-dimensional object repeats the processing steps of: fixedly placing a planar plotting mask between a light source and the surface of a photocurable resin composition, the mask being provided with a liquid-crystal shutter capable of shielding and passing light through microdot areas; forming a predetermined mask pattern on the planar plotting mask in accordance with a cross-sectional profile pattern of one layer to be formed with the planar plotting mask remaining stationary; exposing the surface of the photocurable resin composition to light by way of the mask pattern to thus cure the photocurable resin composition, thereby forming a cross-sectional profile pattern of the layer; supplying a photocurable resin composition of one layer over the optically-cured cross-sectional profile pattern; forming the next predetermined mask pattern on the planar plotting mask in accordance with a cross-sectional profile pattern of one layer to be formed with the planar plotting mask remaining stationary; and exposing the surface of the photocurable resin composition to light by way of the mask pattern to thus cure the photocurable resin composition for the next one layer, thereby forming a cross-sectional profile pattern of the layer.
According to this method, the surface of the photocurable resin composition is exposed to light, thereby forming, in a planar manner, the optically-cured cross-sectional profile pattern of one layer by means of a single operation. Therefore, the optical building speed can be increased when compared with the previously-described conventional method using the spot-shaped UV laser and the method described in JP-A-4-305438, which has been referred to previously and employs the line-shaped exposure mask comprising the optical shutters capable of controlling a shield of light of the microdot areas which are continuously arranged in a line.
When a stereolithographic three-dimensional object is manufactured by this method, an interval between adjacent microdot areas that are projected from the planar plotting mask on the surface of the photocurable resin composition is required to be 0.1 mm or less, from the viewpoint of building accuracy (resolution). Therefore, for instance, a small article whose building area size measures 250 mm by 250 mm requires a number of pixels about 2500 by 2500 dots at least. In the case of a middle-size article whose building area measures 600 mm by 600 mm, the number of pixels required is about 6000 by 6000 dots. However, liquid-crystal masks (liquid-crystal shutters) or digital micromirror shutters, which realize such the high resolutions, are not currently available, or extremely expensive even if available.
Under the method for effecting exposure with the fixedly-positioned planar plotting mask being stopped, the degree of accuracy of an exposed profile pattern is determined by the degree of accuracy (roughness) of the planar plotting mask and the scale-up or scale-down factor of a pattern projected on the surface of the photocurable resin composition by way of the planar plotting mask. The smaller the scale-up factor (the greater the scale-down factor) is set, the smaller the distance becomes between optical dots on the surface of the photocurable resin composition, so that the degree of accuracy of the cross-sectional profile pattern is enhanced. Meanwhile, the greater the scale-up factor is set, the greater the distance becomes between the optical dots on the surface of the photocurable resin composition, so that the degree of accuracy of the cross-sectional profile pattern is decreased.
Therefore, in the case of this method under which the planar plotting mask is fixedly arranged, difficulty is encountered, under the present circumstances, in manufacturing a large-size, stereolithographic three-dimensional object having an enhanced degree of accuracy (building accuracy), and the method can be applied solely to manufacture of a small-size stereolithographic three-dimensional object, in view of the degree of accuracy (building accuracy).
With a view toward solving the drawbacks in the method using a fixedly-arranged planar plotting mask and enabling manufacture of a large-size, stereolithographic three-dimensional object by use of a small-size liquid-crystal shutter, the following method has been proposed (JP-A-8-112863). This method is for manufacturing a stereolithographic three-dimensional object by repeating the processing steps of: arranging a liquid-crystal shutter (a liquid-crystal mask), which selectively passes and shields transmission of light, so as to allow the light to travel in parallel with the surface of a photocurable resin liquid; dividing a travel range of the liquid-crystal shutter into a plurality of sub-divided areas; moving the liquid-crystal shutter to a sub-divided first area of the travel range and stopping the shutter in that position; radiating light from a light source provided on the back of the liquid-crystal shutter, the shutter remains stationary, to the surface of the photocurable resin by way of the liquid-crystal shutter while the light source is being shifted within the range of the liquid-crystal shutter, to thus form a cured area corresponding to the sub-divided first area; moving the liquid-crystal shutter to a sub-divided second travel area and stopping the liquid-shutter in that position, and radiating light from the light source provided on the back of the liquid-crystal shutter, the shutter remains stationary onto the surface of the photocurable resin by way of the liquid-crystal shutter while the light source is being shifted within the range of the liquid-crystal shutter, to thus form a cured area corresponding to the sub-divided second area; performing the same operation until a predetermined cross-sectional profile pattern of one layer is formed on the surface of the photocurable resin composition; and repeating these processing steps until a predetermined stereolithographic three-dimensional object is formed. However, in the case of the method described in JP-A-8-112863, a cured cross-sectional profile pattern of one layer is formed by means of repeating operations; namely, operation for moving the liquid-crystal shutter to the sub-divided first travel area; operation of exposing the surface of photocurable resin with the liquid-crystal shutter remaining stationary (forming an optically-cured area on the surface of photocurable resin); operation of moving the liquid-crystal shutter to the sub-divided second travel area; and operation of exposing the surface of the photocurable resin with the liquid-crystal shutter remaining stationary (forming an optically-cured area on the surface of the photocurable resin). A stereolithographic three-dimensional object is manufactured by means of repeating these operations for a plurality of layers. When the liquid-crystal shutter has already moved to each of the plurality of sub-divided travel areas, radiation of light is not carried out. Therefore, under this method, exposure is performed not continuously but intermittently, and hence building speed becomes slow. Further, under this method, the travel range of the liquid-crystal shutter is divided into a plurality of sub-divided areas, and the photocurable resin composition is cured with the liquid-crystal shutter remaining stationary in each of the sub-divided areas. The cured state is likely to become discontinuous or non-uniform in a border between the sub-divided travel areas. As a result, unevenness in the strength of the entire stereolithographic three-dimensional object, insufficient strength, unsatisfactory appearance, and a drop in dimensional accuracy are likely to arise.